Build for only one target for docs.rs (#5427)

* Add docs.rs metadata to all cargo.toml files

* Remove docs.rs metada in substrate's cargo.toml

substrate/primitives/runtime-interface/proc-macro/Cargo.toml

@@ -18,3 +18,6 @@ quote = "1.0.3"
 proc-macro2 = "1.0.3"
 Inflector = "0.11.4"
 proc-macro-crate = "0.1.4"
+
+[package.metadata.docs.rs]
+targets = ["x86_64-unknown-linux-gnu"]